1. Field
The disclosed embodiments relate to the field of safety of the systems that comprise a structure made of insulating material likely to be subjected to lightning strikes. In particular, the disclosed embodiments relate to a device and a method for determining the conditions in which an insulating structure, for example an aircraft radome, is subjected during its use to one or more lightning strikes.
2. Brief Description
The structures exposed to atmospheric conditions, for example those of aircraft, are frequently struck by lightning when the atmospheric conditions are stormy, in particular when aircraft are flying in such conditions. This situation, even if spectacular, is not critical to safety. In the case of aircraft, which will be used as a basis for the rest of the explanation, as in the case of complex systems in general, the integrity of the equipment, electronic in particular, and the safety of the aircraft and of the passengers are not affected because of the precautions taken when designing modern aircraft.
In fact, on the one hand, the structures of the aircraft are generally metallic or, when they are made of a non-conductive material such as an organic compound material, they include electrically conductive materials, as described, for example, in the patent published under the number FR 2 582 987 or even the patent published under the number FR 2 720 214, such that the structure constitutes a Faraday cage around the passengers of the aircraft and the onboard equipment and, on the other hand, everything is done in the design of the aircraft to enable the electrical charges to flow and to be dispersed into the outside air.
However, certain parts of the aircraft, in particular the antenna protections, or radomes, cannot be covered with electrically conductive materials because of their function which requires a radiofrequency transparency that is as pure as possible, which a conventional metallization could not provide.
These radomes 1, as illustrated in FIG. 1, of generally convex shape for aerodynamic reasons and, for example, made of a compound material using a silicon fiber-reinforced organic resin, are often located in areas in which the lightning is preferably attracted to the aircraft, for example the front part of an airplane fuselage 2 or a nacelle under a wing.
The effects of a lightning strike on a structure which is not able to rapidly dispel the energy of the lightning, like an electrically insulating radome structure, are known. The structure concerned can be greatly damaged and even locally destroyed.
To limit the consequences of the lightning strikes on the structure of the radomes, without unacceptably compromising the radiofrequency transparency, many radomes are provided with strips 10 made of electrically conductive material, of small width, linked to the main electrically conductive structure 20 of the aircraft. These strips, called lightning arrester strips, are used to dispel the surface electrical charges that would tend to accumulate on the insulating surface of the radome because of the friction of the aircraft on the air and make it possible to direct the lightning currents to the main structure 20 of the aircraft with a minimum of electrical resistance.
However, a radome may be damaged by a lightning strike.
When this situation occurs in flights intended to test a new radome model, it is very difficult to understand the exact causes of the damage to the radome observed after a flight and in particular to apportion the consequences associated with specific radome characteristics (shapes, materials, etc.) and those associated with the characteristics of the lightning strike or strikes suffered. This separation of the causes is all the more difficult to achieve given that the lightning strike conditions created in a laboratory can sometimes lack representativeness compared to those encountered in a real situation in a flight and that the number of impacts during the flight, the lightning strike points and the intensities of each impact are not known.
When this situation is encountered during an operating flight of an aircraft, the damage to the radome can cause a flight to be interrupted or certain operational performance characteristics of the aircraft to be limited.
If, as is most common, for example as when the radome 1 forms the nose 21 of an airplane in a front area of the fuselage 2, a pilot is neither able to check the real state of the radome nor able to quantify the energy of the lightning strike, which would make it possible to assess a maximum possible damage to the radome, the pilot is not able to decide precisely on what provisions must be made for the end of the flight and must therefore overestimate the risk as a precaution.
Furthermore, the extent of the damage to the radome will be estimated only after landing by the maintenance teams which will then decide on the repairs to be made, in particular if the radome 1 must be replaced before the next flight of the aircraft. Given the tools needed and the need to requisition a replacement radome from the spares shops, the immobilizing of the aircraft risks penalizing the operator of the aircraft whereas a precise knowledge of the risk by the maintenance teams before the aircraft has landed would have made it possible to anticipate the repair actions to be made and reduce the down-time of the aircraft before the return to flight status.
It is therefore particularly important on the one hand when developing an aircraft that the actual lightning strike conditions to which a radome is subjected are perfectly identified to create an accurate relationship between the observed damage and the characteristics of the lightning strikes and, on the other hand for the operation of an aircraft, for the flight crew to be able, in the event of a lightning strike, to estimate the scale of the possible damage to a radome and for the ground crews in charge of maintenance to be able to anticipate the repair operations even before the aircraft arrives at its destination.